1. Field of the Invention
The present invention relates to a condenser microphone, and in particular to a side-entry condenser microphone which prevents a directional axis from varying depending on a sound source frequency.
2. Description of the Related Art
A side-entry condenser microphone generally used at a studio includes a head case portion 11, a body portion 12 for supporting the head case portion 11 and a connector portion 13 mounted on the lower end side of the body portion 12 as shown in FIGS. 3 and 4. It is to be noted that FIG. 3 is a front sectional view of a conventional side-entry condenser microphone 50 and FIG. 4 is a side sectional view thereof.
A substantially whole part of the head case portion 11 is covered by a metallic guard net 16 which is supported by a frame member 15 formed of a metallic material such as brass.
Further, a condenser microphone unit 17 is disposed in an internal space covered by the guard net 16. As shown in FIG. 3, within the head case portion 11, this microphone unit 17 is supported on both right and left sides by an up-down extending stay 18 and disposed so as to orient its sound collecting axis in a horizontal direction.
Further, the body portion 12 is formed into a cylindrical shape also using a metallic material such as brass, and a circuit board 19 supported by the stay 18 is disposed therein. This circuit board 19 is mounted with an electronic circuit such as an audio output circuit including an impedance converter and a polarization voltage generating circuit.
Still further, the connector portion 13 has such a structure that an output connector 21 is disposed in a cylindrical portion also formed of a metallic material such as brass. Usually, as the output connector 21, an output connector of three-pin type connected to a phantom power source via a balanced shield cable is used.
In such a side-entry condenser microphone 50, the microphone unit 17 and the circuit board 19, and the circuit board 19 and the output connector 21 are electrically connected to each other by respective wirings (not shown), and the head case portion 11 for accommodating them, the body portion 12, and the connector portion 13 are connected integrally with machine screws.
Such a side-entry condenser microphone structure is disclosed in Japanese Patent Application Laid-Open Publication No. 2007-053522 (Patent Document 1).
Incidentally, in the above-mentioned side-entry condenser microphone 50, a case member (hereinafter referred to as a reflector member 20) is provided so as to cover the circuit substrate 19 and minimize a space for accommodating the microphone unit 17 as illustrated in the drawings.
This reflector member 20 is preferably formed to have a circular dome shape (it may also have a cone shape) in order to prevent generation of irregularities in intensity of sound wave inputted at a predetermined frequency.
However, in the case where the reflector member 20 is provided, since it is close to the microphone unit 17, there is a problem in that a location of sound terminals provided at front and rear positions of the microphone unit 17 is moved to an opposite side of the reflector member 20 with respect to the sound collecting axis, i.e. moved in the upward direction, thereby causing a directional axis in a vertical plane to be shifted in the upward direction (see arrows in FIG. 4).
This problem can be observed in a polar pattern illustrated in FIG. 5 showing a directivity of a microphone measured by the present applicant by use of a side-entry condenser microphone having the structure illustrated in FIG. 3.
Further, since the reflector member 20 has a predetermined dimension, a shifting amount of the directional axis depends on the frequency. As shown in a graph of FIG. 6 (a directional frequency response of FIG. 5), the shifting amount becomes small at lower frequencies and large in higher frequencies.
That is, in the structure of the conventional side-entry condenser microphone as shown in FIGS. 3 and 4, there is a problem in that the directional axis varies depending on the sound source frequency.